Neurons of the enteric nervous system (ENS), i.e. the enteric neurons, are located within the gastrointestinal (GI) tract, pancreas, and gall bladder. They control the movement, secretion, and absorption of the digestive system. Deficiency of enteric neurons causes abdominal distention and constipation, a condition of patients suffering from the Hirschsprung's disease. The receptor tyrosine kinase Ret is critical for ENS formation and mutations in RET have been associated with the Hirschsprung's disease. Recent studies showed that the protein product of the Growth arrest specific gene 1 (Gas1) could bind to Ret and modulate Ret signaling. My laboratory studies the role of Gas1 during mouse development. This proposal is based on our observation that Gas1 mutant mice have defects in the number, position, and axonal projection of enteric neurons. We have previously shown that Gas1 facilitates Sonic Hedgehog (Shh) signaling. The Shh mutant has similar defects in enteric neurons as the Gas1 mutant. We propose to test the hypothesis that Gas1 mediates Shh and Ret signaling to direct the positioning and axonal projection of enteric neurons. In Aim 1, we will define the defects of the GI tract in the Gas1 mutant. In Aim 2, we will examine whether the Gas1 mutant ENS has altered Shh and/or Ret signaling. In Aim 3, we will determine whether Gas1 mediates a cross-talk between Shh and Ret signaling using in vitro explant and enteric neurospheres. We will test the hypothesis that Gas1 mediates Shh-directed inhibitory activity to keep enteric neuron cell bodies and their axons away from the gut epithelium, where Shh is expressed. We will also test whether Gas1 negatively regulates downstream effectors of Ret in the ENS. Our goal is to test the hypothesis that Gas1 modulates Shh and Ret signaling to control enteric progenitor/neuron positioning and axon arborization.